1. Field of the Invention (Technical Field)
The present invention relates to optical sensors.
2. Background Art
Fiber optic sensors have been used for analytical purposes for a number of years. In their usual realizations, they use either the optical and physical properties of the fiber core material or that of the lower index internally reflecting cladding to develop a response. Responses that have been used include: direct optical absorption of transmitted light by the core material; absorption produced in the reflective cladding material that attenuates the transmitted light in the core by reducing interface reflectivity; changes in the refractive indices of the cladding or core leading to changes in light transmission sensed by interferometry; or the development of fluorescence in the core material or some reagent situated at the core terminus.
In some of these realizations, strain induced by sorption of the analyte in either the cladding or the core lead to responses measured interferometrically. Fiber optic strain gauges have been constructed in which the material properties are altered mechanically.
Generally, fiber optic sensors are constructed by altering the optical core, the cladding material, or sometimes even the outer protective layer to give an optically detected response in some defined region of the optical fiber""s length. Sensors that result in a modification of the core material""s optical properties are the most sensitive, but are also the most difficult to manufacture. Doping the core with a sensing reagent that indicates an environmental change necessitates fabricating the light guide from doped, bulk material and providing a cladding that transmits the analyte of interest. U.S. Pat. No. 5,286,777, to Schoeler et al., entitled xe2x80x9cPreparing a dye-containing polymer,xe2x80x9d discusses a method for producing doped core material for incorporation into a fiber optic sensor element. Changes in the optical properties of the sensing reagent induced by the environmental analyte are then measured as changes in light transmission of the fiber.
As mentioned above, custom fabrication of the fiber optic core can be difficult and consequently drives up sensor costs. Utilizing sensing reagents incorporated in the cladding material is considerably less expensive, and works by altering the optical properties of the fiber by a process called xe2x80x9cfrustratedxe2x80x9d or xe2x80x9cattenuatedxe2x80x9d internal reflection. These sensing layers are directly exposed to the environment and can be applied after fiber manufacture. Smardzewski (Talanta, Vol. 35, No.2, pp. 95-101 (1988)) discusses such a sensor in which the fibers are replaced by optical waveguides externally coated with an analyte sensitive cladding. U.S. Pat. No. 5,268,972, entitled xe2x80x9cAromatic Hydrocarbon Optrodes for Groundwater Monitoring Applications,xe2x80x9d to Tabacco, et al., issued Dec. 7, 1993, discusses a sensor constructed with porous cladding material whose refractive index is modified by absorption of aromatic hydrocarbons resulting in reduced transmission by attenuation of the sensing light. However, choices of sensing reagent are limited by the rigorous optical requirements for cladding materials. For most organic fibers or fused silica, cladding with either fluorocarbons or silicones is necessary to provide a lower index of refraction. These materials are poor solvents for most complex organic sensing reagents or analytes.
In addition to the aforementioned materials considerations, there are other issues involved in fiber optic sensor production. Often, special fibers (either modified core or modified cladding) must be produced for the active sensor region and then coupled to inactive fiber optic lead(s) following production. The coupling of the sensing segment to the leads often is a limiting factor in sensitivity, reproducibility or device fabrication. Typical of this type of sensor, is the xe2x80x9cReal Time Sensor for Therapeutic Radiation Deliveryxe2x80x9d, U.S. Pat. No. 5,704,890, to Bliss, et al., issued Jan. 6, 1998. In this device, the fiber optic leads are coupled to a scintillator (or scintillator segments) and serve to collect and transmit the scintillations to the detector. A limitation of this device is the necessity to carefully adjust the refractive indices of the scintillator material to that of the fiber optic leads to ensure efficient collection of light. Guthrie et al. (Talanta, Vol. 35, No. 2, pp. 157-159 (1988)) also describe an extrinsic sensor where the sensing element is a film sensitive to the pH of the solution in which it is immersed. The fiber optic leads serve to read changes in the color of the pH sensitive film.
Device calibration may also be an issue. U.S. Pat. No. 5,307,146, entitled xe2x80x9cDual-Wavelength Photometer and Fiber Optic Sensor Probe,xe2x80x9d to Porter, et al., issued Apr. 26, 1994, and U.S. Pat. No. 5,446,280, entitled xe2x80x9cSplit-Spectrum Self-Referenced Fiber Optic Sensor,xe2x80x9d to Wang, et al., issued Aug. 29, 1995, teach the necessity to analyze the sensing light at more than a single wavelength in order to achieve long term stability and calibration of the sensor. This places additional requirements on the sensor optical properties if the sensing and reference functions are confined to a single optical waveguide core. U.S. Pat. No. 5,563,967, entitled xe2x80x9cFiber Optic Sensor Having a Multicore Optical Fiber and an Associated Sensing Method,xe2x80x9d to Haake, issued Oct. 8, 1996, uses two optical waveguides in a single fiber cable to overcome this problem in a device to measure mechanical differences between the sensor and reference elements.
The aforementioned references collectively contain a variety of limitations. Some require complex measurement systems while others require features that limit noninvasiveness. Therefore, a need exists for sensors that are easily instrumented and can operate in a relatively noninvasive manner.
The present invention is of an optical waveguide sensor comprising a waveguide core; a reflective cladding; and at least one intermediate layer positioned between the waveguide core and the reflective cladding comprising a material responsive to at least one environmental stimulus; and wherein refractive indices of the core (n1), the at least one intermediate layer (n2) and the reflective cladding (n3) obey the relationship n2xe2x89xa7n1 greater than n3. The at least one intermediate layer comprises a material that produces a response to at least one environmental stimulus that is detectable by electromagnetic absorption and/or electromagnetic transmission. The environmental stimulus is, for example, chemical concentration, ultraviolet radiation and/or ionizing radiation.
The waveguide sensor of the present invention can comprise a segment of a waveguide. In most instances, a waveguide comprises a waveguide core and a reflective cladding; however, any medium in contact with the fiber core that has a refractive index according to n3 of the above-mentioned equation will act, to some degree, as a reflective cladding. For example, when a fiber core is immersed in a medium having a refractive index that is less than that of the fiber core, a wave internal to the fiber core will have a pronounced reflective component. Of course, the present invention is not limited to use of a xe2x80x9cfiberxe2x80x9d and it is understood that other waveguide geometric configurations are possible, such as, but not limited to, planar waveguides. Furthermore, waveguide sensors of the present invention are useful in a variety of geometric operational configurations, such as, but not limited to, transmission and reflective configurations. In transmission operational configurations, the sensor comprises, for example, at least one segment of a waveguide. In reflective operational configurations, the sensor comprises, for example, a terminal end of a waveguide. In such a configuration, the sensor comprises a reflective cap and at least one intermediate layer positioned between the terminal end of the waveguide core and the reflective cap. As mentioned previously, for operation in a medium having a low refractive index (compared to that of the waveguide core), the need for a reflective cladding is reduced or eliminated. In such instance, a preferred embodiment of the present invention comprises a waveguide core comprising an outer surface and a layer positioned on the outer surface of the waveguide core wherein the layer comprises a material responsive to at least one environmental stimulus. The layer further an outer surface that contacts a medium comprising a refractive index greater than the refractive index of the waveguide core. As for embodiments of the present invention comprising a reflective cladding, the material responsive to at least one environmental stimulus produces a response detectable by, for example, electromagnetic absorption and/or electromagnetic transmission. The material is typically responsive to environmental stimulus such as, but not limited to, chemical concentration, ultraviolet radiation and ionizing radiation. Of course, the sensor may comprise more than one intermediate layer, and such layers may produce responses to the same or different environmental stimuli. Sensors of this particular embodiment are useful in the aforementioned geometric configurations and geometric operational configurations.
Sensors of the present invention may be made according to a method of the present invention for fabrication comprising: providing a waveguide core having an outer surface and affixing an intermediate layer to the outer surface of the waveguide core wherein the intermediate layer comprises a material responsive to at least one environmental stimulus. In certain instances, the method further comprises an additional step of affixing a reflective cladding to the outer surface of the intermediate layer. The method of the present invention is useful for fabricating sensors wherein the material responsive to at least one environmental stimulus produces a response detectable by at least one member selected from the group consisting of electromagnetic absorption and electromagnetic transmission; wherein the material responsive to at least one environmental stimulus produces a response to at least one environmental stimulus selected from the group consisting of chemical concentration, ultraviolet radiation and ionizing radiation; wherein the sensor comprises a segment of a waveguide wherein the waveguide comprises a waveguide core and a reflective cladding; and wherein the sensor comprises a terminal end of a waveguide wherein the waveguide comprises a waveguide core comprising a terminal end and a reflective cladding. For reflective operational configurations, the method comprises steps for affixing a reflective cap such that at least one intermediate layer is positioned between the terminal end of the waveguide core and the reflective cap.
The present invention provides for insertion of at least one cladding layer between a reflective cladding and an optical core material to induce optical absorption directly in the transmitted spectrum. Embodiments comprising an added single cladding are referred to herein as having a dual cladding configuration. Preferred embodiments of the present invention are constructed on a preformed fiber optic light guide without the necessity for connectors to the sensor segment. Optical properties for an intermediate, or xe2x80x9ccladdingxe2x80x9d, layer only require a refractive index equal to or greater than that of the original core material. Under this condition, an added intermediate layer becomes part of the optical core, which together form a sensing region. The optical properties of a fiber optic waveguide containing such a sensing region xe2x80x9csegmentxe2x80x9d are described below.
A primary object of this invention is to provide a simple optical waveguide configuration for the construction and manufacture of optical waveguide sensors.
A further object of this invention is to provide a means to render preformed optical waveguides sensitive to environmental components.
Yet another object of this invention is to provide a means for producing optical waveguide sensors without necessity for incorporating connectors between a sensor segment and an optical lead.
An additional object of this invention is to provide sensor configurations sufficiently small to be inserted into a local environment without excessive perturbation of that environment.
The present invention provides several advantages over previously used fiber optical waveguide sensors. Among these are: ease of fabrication on existing, preformed optical fiber; separation of the required physico-chemical properties of the sensor and cladding layers into distinct host polymers; and absence of connectors to attach an active portion of a sensor to at least one optical lead necessary for connection to a remote read-out device. In combination, these advantages reduce production costs of sensors and increase operational flexibility.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.